The present invention relates to a serial printer or typewriter having a petal-type print thimble, and more particularly to a character selecting mechanism which selects that one of the characters to be printed from sets of characters arrayed on upper and lower positions on the circumferential surface of the print thimble by enabling the print thimble to rotate around its vertical axis and to shift the vertical direction.
Such a serial printer or typewriter comprises a petal-type print thimble having a set of elastic fingers disposed in the form of petal. A plurality of characters are arrayed along plural outer circumferences of the set of elastic fingers. Namely, the characters are arranged on upper and lower positions on circumferences of the print thimble. Such a print thimble is disclosed in U.S. Pat. Nos. 4,389,126 and 4,509,872. In order to select one of the characters, the serial printer has a character selecting mechanism by which the printer thimble is rotated in a horizontal direction and is also shifted in a vertical direction. The character to be printed is located between the paper on the platen and a print hammer by the character selecting mechanism.
The serial printer disclosed in U.S. Pat. No. 4,389,126 comprises a print thimble which is shiftable along the shaft of a rotary motor for rotating the print thimble. A torque piece is fixed to the motor shaft and a vertical slide sleeve is slidably coupled to the motor shaft in the axial direction of the motor shaft. The vertical slide sleeve engages the torque piece so as to be rotated together with the torque piece by the rotary motor. Further, the vertical slide sleeve engages cam means provided on the rotary shaft of a vertical motor. The vertical motor shifts the vertical slide sleeve in the vertical direction. The print thimble is mounted on a torque disk provided on the vertical slide sleeve, so as to be rotated in the horizontal direction by the rotary motor and shifted in the vertical direction by the vertical motor.
The character selecting mechanism requires a torque piece and a vertical slide sleeve, which are structurally complicated and difficult to manufacture. Further, since the print thimble is not directly secured to the shaft of the rotary motor, the rotational positioning of the print thimble is slightly rotationally deflected with respect to the rotational angle of the rotary motor.
The printer disclosed in U.S. Pat. No. 4,509,872 includes a rotary motor having an axially movable shaft. The rotor of the rotary motor is shorter than the stator in the axial direction. The print thimble is attached to the upper end of the shaft. The other end of the shaft engages a plane cam driven by a vertical motor. The vertical motor rotates to vertically shift the rotary motor shaft via the plane cam so as to displace the print thimble to the upper or lower position. This character selecting mechanism requires an expensive rotary motor due to its special structure with the result that the manufacturing cost of the printer is increased.
Further, in both of the above-mentioned character selecting mechanisms, the rotary axis of the print thimble is disposed on an extension of the shaft axis of the rotary motor. Therefore, these mechanisms require a large space in the vertical direction. Furthermore, since no mechanism is provided for locking the print thimble in the rotational direction during the printing operation, the print thimble may rotate slightly owing to the impact of the print hammer, and the selected character will be slightly deflected. As a result, the printing quality is poor.